Integrated photonics products include filters, add-drop multiplexers, mirrors and many other optical and photonic devices for optical telecommunications, photonic interconnects, and optical sensing. Integrated devices can be built up in layers with lower devices being built first and then more devices being built up over the lower devices. In microelectronics, the devices on lower layers can be tested before more devices are layered on. Many microelectronic designs include contact pads to allow test probes to electrically connect with a partially finished wafer. If the circuits do not perform well, they can be repaired or discarded. This avoids the waste of building up higher layers only to discover later that the circuits are defective.
In optical devices, gratings are commonly used instead of electrical contact pads to couple light in and out of an optical circuit. Etched grating couplers are often used for testing prototype photonic devices, although no standard fabrication and testing technique currently exists for integrated photonic devices.
Although etched gratings can be efficiently used to couple light in photonic devices for testing and characterization purposes, once fabricated, they are permanently embedded in an optical circuit.
Typically, an intermediate grating coupler will disconnect the optical paths on either side or reduce the efficiency at which the light propagates in the optical circuit. Since the grating is an alteration of the wafer surface, the grating must be removed physically from the wafer after device testing, or the optical pathways must be re-routed to avoid the test gratings. This limits the use of the etched gratings couplers for in line optical testing and adds expense in removing them.
Photonic and other optical devices can be mass produced on a silicon wafer using processes that are well established for microelectronics, most notably by using CMOS (Complementary Metal Oxide Semiconductor) compatible fabrication techniques based on the silicon on insulator (SOI) material system. Alternatively integrated optical devices can be made using other materials such as lithium niobate, silica and III-V processes.